Modern cable systems utilize a hybrid fiber cable (HFC) architecture in which signals are distributed via a fiber optic connection to a node that converts the optical signal to an electrical signal and distributes the signals to residences (subscribers) via a tree and branch coaxial cable distribution network (“plant”) consisting of the coaxial cable, amplifiers and taps. The plant can be made bidirectional through the use of a fiber optic return signal from the node to the head end. A return band, typically from 5–42 MHz, is used to support transmission from devices in the residence back to the head end. Transmission from the residences are received at the node, converted to an optical signal, and transmitted to the head-end on a separate return fiber or on a return wavelength separate from the downstream wavelength.
Suppressing undesirable energy in an HFC network, particularly ingress noise in the HFC upstream, is an important characteristic when operating a network having such a bi-directional communication path on a shared wire between a head-end and each of a plurality of remote points. One technical challenge is to maintain adequate network integrity for signals being transmitted in the return path so that the information in these signals is not contaminated and does not either require retransmission (if data traffic), or is defined as “dropped” (if voice traffic). “Ingress” is defined as unwanted energy that enters the network at a weak point, where these weak points are all too often at or near a remote point where there is a shield discontinuity, a poor shield, or a faulty connector. For example, two-way dispatch services, amateur radio transmission, various commercial, medical or industrial electronic equipment, as well as ignition noise from combustion engines, all contribute to ingress noise. Additionally, one very common and troublesome source of ingress noise is electromagnetic emissions at a subscriber's premise from electric motors in fax machines, vacuum cleaners, hair dryers and the like. These emissions are often coupled onto the cable system cable via unterminated cable stubs in the subscriber's premise, the stubs tending to act as antennas. Thus, the upstream “ingress” noise signals will sum at the head end from the multiple weak points in both the plant and the subscriber premises.
It is desirable for a system operation to be able to mitigate upstream ingress noise originating at the subscriber premises, while minimally disturbing service to the subscriber.